1. Field of the Invention
The present invention relates to an optical data medium for recording and reproducing data by the use of a laser beam and a method for making the same. The present invention also relates to a cassette casing for carrying the optical data medium.
2. Description of the Prior Art
Optical data media in such forms as optical cards and disks are now commercially available with optical disks in particular being widely distributed as compact discs and laser discs. Higher data densities are desirable for a variety of reasons, including the ability to make CDs smaller and to enable the recording of High Definition Television (HDTV) compatible video discs.
One method considered possible for achieving a high data density in an optical data medium was to focus the read laser to an even finer beam diameter, but the maximum practical limit to the numerical aperture (NA) of the objective lens which focuses the laser is approximately 0.6. It would also be possible to obtain a similar result by shortening the laser wavelength, but a wavelength of a practical semiconductor laser is limited to approximately 670 nm for the present. A second method is a new method whereby a high density is proposed to be achieved using V-grooves in the optical disk, such as disclosed in U.S. Pat. No. 4,569,038 issued Feb. 4, 1986 to Nagashima et al.
The capacity of the data that can be achieved on an optical disk using conventional technologies is determinable. For an optical disk housed in a 3.5 inch cassette case, the disk diameter is approximately 86 mm, providing a data storage area with a radius of 25 mm to 41 mm. It is assumed that a 670 nm wavelength laser is used with a 0.55 NA objective lens. With a 0.8 .mu.m track pitch (1.6 .mu.m V-groove pitch) and 0.47 .mu.m bit length, the unformatted single-side capacity is 1.1 Gbytes. Using the standard CD format, a single-side formatted capacity of 550 Mb, the same as a 12 cm CD, can be obtained, but this is currently the maximum limit.
If information conventionally provided as printed material is to be provided in the future on a CD-ROM or other optical data medium, it will also be necessary and convenient if the user is able to write to the medium to record notes as is done currently with paper media, instead of only being able to read the information. One method which makes this possible divides the disk into a read/writable areas, wherein a recording film is formed on the inside portion of the disk, and a read-only area comprising a reflective metal film on the outside portion of the disk. However, this method reduces the read-only data area, and therefore limits the amount of information distributable on a single disk.
As thus described, while smaller, higher capacity optical data media, including optical disks and cards, are desirable, current technologies are already at or near the recording density limit. In addition, there is a demand for a read-only data distribution medium in which an area which can be written to by the user is provided with a minimal decrease in the read-only area.
In addition, CDs, video discs, and similar optical discs are not housed in a cassette case as are floppy disks and other magnetic data media. However, while optical disks are read with a non-contact pickup, the playback signal quality can still be degraded by oil and other foreign matter deposited on the surface by, for example, touching the surface with the fingers during disk loading and unloading. As a cassette case typically has a shutter which remains closed and hides the disk surface at all times other than during playback, it is now considered necessary and preferable to house optical discs in a cassette case.